The present invention pertains to a scratch measuring method and apparatus and more particularly to a portable scratch measurement system which provides a visual display of a scratch together with information regarding scratch depth.
There are many situations where it is desirable to measure the depth of the scratch on a surface. For example, in airplane surfaces scratches can cause structural problems, or, in the case of a stealth-type aircraft, scratches in the surface coating may reduce the stealth effectiveness, when the scratches extend beyond a certain depth. Various methods have been employed heretofore to measure scratch depth. For instance, chemical methods have been employed, but are undesirable in that they can be disorderly and dangerous, employing hazardous materials which give rise to safety and environmental concerns. Further, the chemical processes typically affect areas adjacent the scratched portion. Eddy current testing is an alternative but is unsuitable when the surfaces are formed of non-metal or composite materials. Ultrasound typically requires that sensors be placed on the opposite side of the surface containing the scratch and this method may therefore be impractical. Moreover, reflected ultrasound is often not effective because a rough scratch bottom will provide inconsistent readings.
Optical microscopes have been utilized in the past to view the extent of a scratch, but the equipment can be bulky and difficult to operate, especially in locations requiring an operator to assume an awkward position underneath a plane fuselage. Furthermore, measurement is subjective, based upon the operator's interpretation of the view provided. Mechanical probes are undesirable for scratch measurement inasmuch as the probe can become embedded in the surface, worsening the scratch by increased penetration. Further, the radius of the probe tip may be larger than the scratch width and hence not reach the bottom of the scratch. Other mechanical methods include the use of replication putty together with an optical comparator. Such a method is labor-intensive and the putty may not form into the crack, or may remain behind in the crack and complicate repair. Some radiographic methods are theoretically possible, but it may not be feasible to position a sensor on a back surface of a scratched area. Also, backscatter methods of radiographic testing are expensive.